99 resultados para 680
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For this sake, the macroscopic equations of mechanics and the kinetic equations of the microstructural transformations should form a unified set that be solved simultaneously. As a case study of coupling length and time scales, the trans-scale formulation
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本文从分析多晶金属材料的细观组织在弹塑性变形中的贮能和耗能机制入手,提出一个三维组集式弹塑性本构模型。该模型将材料单元抽象成沿三维空间各方向均匀分布组件的集合体,方向组件反映材料的细观性态并在宏观上协调变形,所有方向组件的内力总效应构成宏观应力。文中导出了显式的弹塑性本构关系,并与Budiansky的复杂加载试验结果及其它塑性模型进行了对比。
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From observed data on lithospheric plates, a unified empirical law for plate motion,valid for continental as well as oceanic plates, is obtained in the following form: The speedof plate motion U depends linearly on a geometric parameter T_d, ratio of the sum of effectiveridge length and trench arc length to the sum of area of continental part of plate and total areaof cold sinking slab. Based on this unified law, a simple mechanical analysis shows that, themain driving forces for lithospheric plates come from push along the mid-ocean ridge andpull by the cold sinking slab, while the main drag forces consist of the viscous traction beneaththe continental part of plate and over both faces of the sinking slab. Moreover, the specific-push along ridge and pull by slab are found to be of equal magnitude.
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This book elucidates the methods of molecular gas dynamics or rarefied gas dynamics which treat the problems of gas flows when the discrete molecular effects of the gas prevail under the circumstances of low density, the emphases being stressed on the basis of the methods, the direct simulation Monte Carlo method applied to the simulation of non-equilibrium effects and the frontier subjects related to low speed microscale rarefied gas flows. It provides a solid basis for the study of molecular gas dynamics for senior students and graduates in the aerospace and mechanical engineering departments of universities and colleges. It gives a general acquaintance of modern developments of rarefied gas dynamics in various regimes and leads to the frontier topics of non-equilibrium rarefied gas dynamics and low speed microscale gas dynamics. It will be also of benefit to the scientific and technical researchers engaged in aerospace high altitude aerodynamic force and heating design and in the research on gas flow in MEMS
[1] Molecular structure and energy states | (21) | ||
[2] Some basic concepts of kinetic theory | (51) | ||
[3] Interaction of molecules with solid surface | (131) | ||
[4] Free molecular flow | (159) | ||
[5] Continuum models | (191) | ||
[6] Transitional regime | (231) | ||
[7] Direct simulation Monte-Carlo (DSMC) method | (275) | ||
[8] Microscale slow gas flows, information preservation method | (317) | ||
[App. I] Gas properties | (367) | ||
[App. II] Some integrals | (369) | ||
[App. III] Sampling from a prescribed distribution | (375) | ||
[App. IV] Program of the couette flow | (383) | ||
Subject Index | (399) |
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The damage morphologies, threshold fluences in ZnO films were studied with femtosecond laser pulses. Time-resolved reflectivity and transmissivity have been measured by the pump-probe technique at different pump fluences and wavelengths. The results indicate that two-phase transition is the dominant damage mechanism, which is similar to that in narrow band gap semiconductors. The estimated energy loss rate of conduction electrons is 1.5 eV/ps. (c) 2005 Elsevier Ltd. All rights reserved.
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报道了利用声光振幅调制锁模的方法,在激光二极管端面抽运Nd:YVO4激光器上获得320MHz高重复频率脉冲列的实验结果。实验采用平一平腔结构,腔长452mm,耦合输出镜透过率为3.6%。所用声光介质为熔融石英晶体,以铌酸锂作换能器,在驱动功率4.5W时,对1064nm波长衍射效率为50,相应的调制深度为0.31。在最佳锁模状态下,激光二极管抽运功率为3.5W,此时激光平均输出功率为15mw。示波器记录脉冲宽度680ps,实测光束质量因子M^2小于1.5。并在实验基础上对激光器工作的稳定性进行了分析,结果表
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采用Pound-Drever-Hall技术,对用于多普勒测风雷达的种子注入激光器的主动激光器进行稳频,将其频率锁定在一个特殊设计的法珀腔上。该法珀腔总体采用零膨胀微晶玻璃材料制成,具有极高的温度稳定性。使用计算机采集鉴频信号并且进行处理。锁定后,1秒内激光器的相对频率漂移为±25kHz,一小时内的相对频率漂移为±55kHz,满足多普勒测风雷达的要求。
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YAlO3 (YAP) crystals with different Yb3+ concentration have been grown by Czochralski method and cooperative fluorescence of Yb3+ ions in YAP crystal was studied under 940-nm infrared (IR) LD excitation at room temperature. The Yb concentration dependence of absorption intensity of IR and charge transfer bands exhibit different features. The green emission band in the region of 480-520nm was assigned to the cooperative deexcitation of two Yb3+ ions. The remaining upconverted emission bands containing various sharp peaks associated with impurity ions were observed and discussed. Charge transfer luminescence of heavily doped 20at% Yb:YAP is strongly temperature dependent and no concentration quenching of the charge transfer luminescence was found through the investigation of different Yb levels samples. (c) 2006 Elsevier B.V. All rights reserved.
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用光纤耦合激光二极管抽运Cr,Yb:YAG晶体获得了1.03 μm的自调Q激光输出,输出的调Q脉冲非常稳定,抽运阈值功率为680 mW,脉冲宽度为3.3 ns,获得的平均功率为156 mW,斜率效率为18.5%。随着抽运功率的增大,重复频率成线性增长,而脉宽略有减少,单脉冲能量和峰值功率都始终呈增大趋势。光束质量因子M2为1.17。
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本文主要研究了真空中有机污染物对薄膜阈值的影响。采用电子枪蒸镀的方式制备了由TiO2/SiO2两种材料组合而成的1064nm高反膜,分别在大气和真空中对其损伤阈值进行了测量。发现在真空中污染物易黏附于光学元件表面,导致损伤;真空中的氧分子会加速损伤过程,根据破斑形貌的分析认为是由于在激光作用下氧分子与有机污染物发生反应造成的;真空中的有机污染物被去除后,真空和大气中的损伤阈值差别不大,破斑形貌相似。结果表明有机污染物是真空中损伤阈值降低的主要原因。